


immanis engorgio

by fifty_fifty



Category: Merlin (TV)
Genre: Alternate Universe - Harry Potter Setting, Alternate Universe - Hogwarts, Alternate Universe - Modern Setting, Anal Sex, Blow Jobs, Bottom Merlin (Merlin), Butt Plugs, Crossovers & Fandom Fusions, Deepthroating, M/M, Magic, Oral Sex, Orgasm Control, Orgasm Denial, Self-Lubrication, Semi-Public Sex, Sex Magic, Size Kink, Size Queen MErlin, Students, Top Arthur Pendragon (Merlin), Top Arthur Pendragon/Bottom Merlin (Merlin), Topping from the Bottom
Language: English
Status: Completed
Published: 2019-09-09
Updated: 2019-09-09
Packaged: 2020-10-13 06:29:00
Rating: Explicit
Warnings: No Archive Warnings Apply, Underage
Chapters: 1
Words: 1,427
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/20577995
Author URL: https://archiveofourown.org/users/fifty_fifty/pseuds/fifty_fifty
Summary: Merlin has learned a new spell as a surprise for Arthur. Though half way through the spell, it becomes apparent that it's not an entirely selfless gift on Merlin's part.





	immanis engorgio

**Author's Note:**

> Written for Kinkalot 2019 Bonus Challenge 2: Crossovers
> 
> This fic is marked this as underaged, as their age in the story could possibly be debatable and considered underage where you are. But I imagine them to be of age in the UK, where this story is set, which is 16.

"Merlin," hissed Arthur. "What are you doing here? More importantly, _how_ did you get in here?"

"Wasn't hard, Arthur," said Merlin nonchalantly. "Most powerful wizard in the school, remember? And besides, you Gryffindor's aren't half as good with spells as you think you are. Most of the time you do well because of boneheaded stubbornness. Which means you were sorted perfectly."

"Merlin, get off my bed and out of here, before we both get into trouble!"

"All right," agreed Merlin. "Meet me in the common room."

And with that he disapparated with a pop, without even uttering a word.

Arthur scrambled out of bed and hurried down the spiral staircase. Sure enough, there was Merlin, slung in a comfy armchair near the fire with his legs draped over the arm, as bold as brass in his Slytherin uniform.

"Nice of you to join me, Pendragon. Still not got the hang of disapparating yet?"

With a scowl, Arthur stood near the other vacant armchair. "You know I can't. No one can on Hogwarts property. Apart from you, apparently. And besides, why bother when I've got you to do all the donkey work?"

"Hey!" protested Merlin. "For that I might just go back to my bed, without giving you your treat..."

Arthur perked up. "Treat?" he asked with a smile.

Merlin nodded and clambered to his feet, a tangle of limbs that shouldn't really work together, but somehow did.

He pushed Arthur down into the soft, worn leather armchair and knelt between his knees. With a wicked grin, he looked up at Arthur, a glint in his eye.

"Do you trust me?" he asked.

Arthur scoffed, despite being a bit nervous. Merlin was always coming up with hare-brained schemes, thinking of any number of things that would get your average Hogwarts student expelled. But they had both yet to be caught. Gryffindor luck, Merlin called it. Slytherin slyness, Arthur would retort with.

"'Course," he replied, his voice hoarse and his mouth dry.

"Good."

Merlin licked his lips and tucked his fingers into the waistband of Arthur's pyjamas, which were already tented with interest.

Arthur should have known. Merlin was such a cock slut. _Of course_ Merlin would disapparate into Gryffindor tower in order to blow him in the middle of the night. Knowing Merlin, he'd probably been working on getting inside for ages, and tonight was the night he had finally succeeded.

"How—how long have you been working on this?" Arthur asked, as Merlin eased Arthur’s pyjama bottoms down his hips, freeing his eager cock.

Merlin tilted his head to the side in thought, his eyes fixated on Arthur’s dick. "About two weeks," he said.

Arthur shook his head fondly at Merlin, who was practically salivating as he wrapped his strong, slender fingers around Arthur's length.

"Oh!" Merlin exclaimed suddenly, pulled out of his thoughts. "That's not all either. Look!"

Merlin smiled as he whispered, "_immanis engorgio_," and his eyes glowed a brilliant gold.

No one had quite been able to figure Merlin out yet. But using wandless magic so intentionally and accurately was unheard of in wizards of their age. Merlin was a magical prodigy.

Arthur yelped and pushed Merlin away, his hands desperately inspecting his cock, which had swollen and grown to at least twice in size.

"Merlin!" he gasped. "What the hell have you done to my penis!? It's—it's _huge_!"

"I know," grinned Merlin.

"Oh, fucking hell," groaned Arthur, unable to take his hands or eyes off of his monstrous new cock.

"Can I..." asked Merlin.

Arthur looked down at him, his breath catching in his throat at the look on Merlin’s face, his eyes huge and dark as he peered up at him.

"Sure," said Arthur. Because he wasn't ever going to turn down a blowjob.

"It's so _big_," Merlin breathed. "It’s better than I even imagined, Arthur."

Merlin batted Arthur's hands away and replaced them with his own. He wrapped his lips around the head, licking and sucking eagerly.

Arthur watched in morbid fascination, his fingers digging into the arm of the chair as he tried his best to be quiet. Merlin slowly started to take more and more of Arthur’s magically massive knob into his mouth. It seemed as though it should be impossible to manage even a quarter of it. But somehow Merlin was already halfway. Arthur felt the head of his cock meet the back of Merlin's throat and Merlin gurgled and choked around him.

Surely that was it. Merlin _had_ to have reached his limit now. But apparently not as Merlin shifted his position and opened his throat even more, and impossibly, he started to deepthroat Arthur.

He made steady progress as Arthur continued watching on. His fingernails bound to leave permanent marks in the leather as Arthur gritted his teeth, determined not to come and spoil their fun.

Merlin moved further and further down his cock, taking more and more until his nose was buried in Arthur's pubes and Arthur was completely enclosed in Merlin's throat. Arthur choked on his own tongue as his thighs trembled and his cock jumped. He was about to— He needed—

Merlin whipped his hand up into the air and Arthur jerked as he was ripped away from the edge of climax. It was as though someone had suddenly squeezed his cock _hard_, encasing it in some kind of ring.

Merlin pulled off, and tried to speak as he massaged his aching jaw. It came out as a hoarse whisper.

"Not yet," he said. "Want to ride you."

"You what!?" Arthur spluttered. "But you can't! There's no way you can take it all."

But his protests fell on deaf ears as Merlin was already up on his feet, stripping out of his uniform; all long legs and pale, cream skin that Arthur ached to touch.

Reaching behind himself Merlin grimaced. He wiggled his hand back and forth, biting back little groans and gasps. Arthur wondered what the hell he was doing until he finally produced a wide tapered butt plug which he cast it aside with his school uniform.

Merlin settled himself in Arthur's lap, legs either side of Arthur’s as he went up on his knees and reached for Arthur's cock. He guided it back towards his stretched hole, and whispered _"inlubrico,"_ as he slowly but surely sank back on it.

"Oh my God," Arthur groaned, covering his eyes with his arm. He couldn’t watch. And yet he must. He peeked underneath his arm to watch Merlin's face. Merlin’s eyes fluttered, colour high upon his cheeks. His teeth worried his bottom lip. before a delicious moan was ripped from them.

Merlin was a beautiful sight to behold. Arthur had no clue why he had picked him of all the students in Hogwarts to be such a kinky perve with, but he was eternally grateful.

He also had no clue whether or not they were boyfriends. But they seemed to have come to some kind of arrangement between them. And Arthur was definitely here for it. All the random meetings around school grounds for secret snogging sessions where they'd get each other off, all the blowjobs... Actual sex sometimes when they could manage it. Which to be honest, was quite often. Merlin loved getting fucked in the forbidden forest. Or in the room of requirement. Or that broom cupboard near Snape's office. Basically, anywhere Merlin thought they could get away with was fair game. Plus Merlin would always turn up by his side on their trips to Hogsmede where Arthur would buy them both butter beers and sweets from Honeydukes. Merlin had a secret weakness for butter beer. Arthur resolved to keep him in constant supply of it once they were older.

It was fair to say that Arthur was fairly smitten.

"Ah, God, Arthur! You're s—so big and g—good," groaned Merlin, gripping Arthur's arms so hard there would be bruises, his legs trembling with effort until he finally bottomed out. He pressed a hand against his stomach which bulged a little and gave Arthur a pleased smile before he started to move, slowly up and down. "This is—This is something else... I—I—"

Merlin wrapped a hand around his own cock, stripping it rapidly. He raised his other hand and swished it downwards. Arthur felt his halted orgasm crash through him, as his mouth fell open in a silent scream. Merlin cried out and clenched around him as his come splattered hot and wet on Arthur's face.

He collapsed against Arthur's chest, exhausted.

"Happy Birthday, Pendragon," he croaked.


End file.
